The invention relates to a method and an apparatus for the automatic positioning of a particle beam to a test spot whose actual position appears shifted relative to its rated position due to local fields.
An electron probe is used for measurements of voltages and voltage progressions in the inside of integrated circuits. The measuring locations are usually metallic interconnects or test spots at the highest interconnect level. In order to reliably position the electron probe in the center of the respective interconnect or of the respective test spot, an automatic positioning of the electron probe is desirable. The dimensions of a test spot or of an interconnect width currently amount to a few .mu.m and will be further reduced in the future. In case the electron probe moves away from the center of the interconnect under investigation, or away from the center of the investigated test spot due to instabilities of the measuring installation employed, or for other reasons, for example as a consequence of electric fields, this movement should be registered and compensated.
The positioning of the electron probe occurs in electron beam measuring instruments (which are usually modified scanning electron microscopes) in a so-called image/point mode. In this image/point mode, the electron beam is alternately swept across the specimen so that an image is generated, and it is then again directed to a specific measuring point. This procedure wherein an image is generated and the electron beam is then directed to a specific measuring point in alternating fashion is repeated so quickly that the specific measuring location is visible on the picture and that, with the assistance of potentiometers which control the voltages for the deflection coils of the scanning electron microscope, the positioning of the electron beam probe can be modified such that the specefic measuring location coincides with a specific measuring point within the specimen An electron beam measuring instrument is disclosed, for example, by U.S. Pat. No. 4,277,679, incorporated herein by reference.
It would be ideal if, similar to usual electron beam lithography, the electron probe would impinge the desired location on the subject with high precision. Such a method for positioning an electron beam is disclosed, for example, by U.S. Pat. No. 3,644,700, incorporated herein by reference. This, however, is impossible in electron beam measurement technology since local fields, such as, for example, local temperature fluctuations or local electrical fields, make the exact position of the respective measuring location appear shifted and thus do not allow the exact position of the respective measuring location to be precisely defined at the outset. The local fields which cause these apparent deviations of the actual position value of a measuring point from the rated position value of this measuring point cannot be exactly defined and are generally also subject to additional chronological fluctuations during a measurement or (in more general terms) during a processing of the specimen.